feat: Allow access to tree::State::buf1|2.

This allows to re-use that memory at least, making this kind of state a little more useful.
Also, these fields can certainly be considered stable.

Author: Byron

gix-diff/src/rewrites/tracker.rs

@@ -168,13 +168,13 @@ impl<T: Change> Tracker<T> {
             return Some(change);
         };
 
-        let relation = change
-            .relation()
-            .filter(|_| matches!(change_kind, ChangeKind::Addition | ChangeKind::Deletion));
         let entry_kind = change.entry_mode().kind();
         if entry_kind == EntryKind::Commit {
             return Some(change);
         }
+        let relation = change
+            .relation()
+            .filter(|_| matches!(change_kind, ChangeKind::Addition | ChangeKind::Deletion));
         if let (None, EntryKind::Tree) = (relation, entry_kind) {
             return Some(change);
         };

gix-diff/src/tree/mod.rs

@@ -36,8 +36,10 @@ pub trait Visit {
 /// The state required to run [tree-diffs](super::tree()).
 #[derive(Default, Clone)]
 pub struct State {
-    buf1: Vec<u8>,
-    buf2: Vec<u8>,
+    /// A buffer for object data.
+    pub buf1: Vec<u8>,
+    /// Another buffer for object data.
+    pub buf2: Vec<u8>,
     trees: VecDeque<TreeInfoTuple>,
     change_id: visit::ChangeId,
 }

gix-diff/tests/diff/rewrites/tracker.rs

@@ -525,17 +525,19 @@ fn rename_by_50_percent_similarity() -> crate::Result {
 #[test]
 fn directories_without_relation_are_ignored() -> crate::Result {
     let mut track = util::new_tracker(Default::default());
-    let tree_without_relation = Change {
-        id: NULL_ID,
-        kind: ChangeKind::Deletion,
-        mode: EntryKind::Tree.into(),
-        relation: None,
-    };
-    assert_eq!(
-        track.try_push_change(tree_without_relation, "dir".into()),
-        Some(tree_without_relation),
-        "trees, or non-blobs, are ignored, particularly when they have no relation"
-    );
+    for mode in [EntryKind::Tree, EntryKind::Commit] {
+        let tree_without_relation = Change {
+            id: NULL_ID,
+            kind: ChangeKind::Deletion,
+            mode: mode.into(),
+            relation: None,
+        };
+        assert_eq!(
+            track.try_push_change(tree_without_relation, "dir".into()),
+            Some(tree_without_relation),
+            "trees and submodules are ignored, particularly when they have no relation"
+        );
+    }
     Ok(())
 }